1. Field of the Invention
The present invention relates to a power supply system for a quadrupole mass spectrometer wherein DC voltages are superimposed on radio frequency (RF) voltages as supply voltages for quadrupole electrodes.
2. Description of Prior Art
In general, a quadrupole mass spectrometer requires, due to its inherent characteristic, supply voltages for its four hyperbolic or cylindrical electrodes, which have been obtained by superimposing DC (direct current) voltages on RF (radio frequency) voltages, so as to spectro-analyze the mass of a sample.
A typical prior art power supply system for a quadrupole mass spectrometer is shown in FIG. 1. In this power supply system, the DC voltages (.+-.U) are superimposed on the RF voltages (.+-.V cos .omega.t) to obtain the superimposed supply voltages (.+-.[U+V cos .omega.t ]). "U" and "V" denote amplitudes of the DC voltages, and RF voltages respectively, whereas ".omega." indicates an angular velocity and "t" represents a time.
This conventional power supply system is known from, for instance, "QUADRUPOLE MASS SPECTROMETRY and its applications" issued by ELSEVIER SCIENTIFIC PUBLISHING COMPANY, AMSTERDAM, THE Netherlands, 1976 by Peter H. Dawson, pages 147, 282 and 295 (see FIGS. 6.19, 11.3, and 12.9).
Operations of the voltage superposition of this power supply system will now be summarized with reference to FIG. 1. In this conventional power supply system, an oscillator 31 produces a reference signal which is supplied to an RF (radio frequency) voltage generator 32. In synchronism with the reference signal produced in the oscillator 31, an RF voltage (.+-.V cos .omega.t) is applied to a superposition circuit 33. Meanwhile a DC voltage generator 35 produces DC voltages .+-.U corresponding to an RF voltages derived from a detection circuit 34 (amplitudes of the RF voltage subdivided from the above RF voltages .+-.V cos .omega.t) and applies these DC voltages to the superposition circuit 33. Then the superposition circuit 33 superposes the RF voltages on the DC voltages to obtain superimposed voltages .+-.(U+V cos .omega.t) which are applied to the electrodes of the quadrupole mass spectrometer 36.
To implement the mass analysis of a sample, ions that are generated from an ion source 37 and to be mass-analyzed are incident upon the quadrupole mass spectrometer 36; a sawtooth wave scanning signal is supplied from a control section 40 to, e.g., the RF voltage generator 32, and the amplitudes of the RF voltages (.+-.V cos .omega.t) to be applied to the quadrupole mass spectrometer 36 are scanned by the sawtooth wave signal under control of the control section 40. In this case, a negative feed-back path in a circuit arrangement constructed by a comparator 41, the RF voltage generator 32 and the detection circuit 34 is usually formed. In this arrangement the DC voltages derived from the detection circuit 34 are superimposed over the scanning signal in the comparator 41. Accordingly, an ion having a predetermined mass number passes through a quadrupole mass spectrometer 36 and is then detected in a detector 38 in synchronism with the scanning signal and finally recorded on a recorder 39 as a mass spectrum. What kind of the ion can be analyzed by analyzing this mass spectrum.
As seen from the circuit diagram of FIG. 1, the DC voltages .+-.U and the RF voltages .+-.V cos .omega.t are separately produced; and thereafter these voltages are superimposed over each other in the superposition circuit 33 so as to generate the desirable analyzing voltages .+-.(U+V cos .omega.t), which are applied to the quadrupole mass spectrometer 36 according to the conventional quadrupole mass spectrometer power supply system. As a result, at least two separate power sources are required to produce the RF voltages .+-.V cos .omega.t and the DC voltages .+-.U.
Moreover, the DC voltages .+-.U must be adjusted to desirable voltages in order to achieve the optimum conditions for the mass spectro-analysis. FIG. 2 shows a conventional controllable DC power supply system. This power supply system employs a positive power source (e.g. +350 V), a negative power source (e.g. -350 V), and two transistors TR.sub.1 TR.sub.2 and desired DC voltages .+-.U are generated in response to the input signal. Accordingly, power supply sources having higher voltages than the desirable maximum DC voltages are necessary. For instance, both positive and negative power sources capable of applying several hundreds of DC voltages are required. In addition, high-voltage controlling transistors are required, resulting in a complex power supply system. This prior art controllable DC power supply system is known from e.g., the quadrupole mass spectrometer, Model AQA-360, ANELVA Corporation, Japan.
An object of the Invention is to prevent the above-described drawbacks of the conventional power supply system, and therefore to provide a power supply system for a quadrupole mass spectrometer without requiring separate DC power sources. Moreover, the power supply system produces the DC voltages (.+-.U) having a specific relation to the RF voltage by processing the RF voltages (.+-.V cos .omega.t), thereby deriving desirable analyzing voltages .+-.(U+V cos .omega.t.)